A suitably tuned laser beam is a practical source of photons for producing isotopically selective excitation of the orbital electrons in a molecular or elemental state material, particularly in a vapor thereof. In one particular application for this technique, as is specifically disclosed in U.S. Pat. No. 3,772,519, specifically incorporated herein by reference, a system is described for using the radiant energy of lasers to produce selective photoionization of one uranium isotope, typically U.sub.235, with respect to the other isotopes of uranium. For this purpose, the uranium is first produced in the form of a vapor, the U.sub.235 particles of which are then laser ionized. The photoionized particles of U.sub.235 are then typically accelerated out of the vapor environment for separate collecting using magnetohydrodynamic forces.
Theoretical analysis of the factors governing selective photoexcitation predicts that in the presence of constant frequency monochromatic radiation, 50% of the available, illuminated atoms in the uranium vapor will be in a photoexcited state, and 50% will be in the unexcited, typically ground state at any given moment. This theoretical limitation is of significance in the planning of production level enrichment processes because of its effect on enrichment yield.
In a further application for the technique of photoexcitation employing laser radiant energy, it is common to cascade one or more stages of laser amplification on the output of a low power laser in order to boost the energy of the laser to higher levels. The lasing condition in each of the amplifying stages typically results from the presence of a "population inversion" wherein particles in a lasing medium have their orbital electrons excited to a predetermined energy level such that a greater percentage of the medium particles are excited to that particular energy level than the proportion of medium particles in a lower lying energy level. These conditions are theoretically necessary for the simultaneous decay of the excited particles to the lower lying energy state which in turn results in the production of laser radiant energy. The power generated by the lasing medium in these circumstances is directly related to the number of excited particles in the medium. In a two-level lasing system then, the same theoretical considerations as mentioned above would limit the number of excited particles to 50% of the available ones and thereby limit the laser output power accordingly.
In addition, in applications where it is desired to selectively photoexcite particles by laser energy, a substantial frequency broadening may exist in the absorption lines of particles which it is desired to selectively excite as, for example, by Zeeman splitting of the energy levels. The presence of this splitting, or the spreading of the original energy level into several levels covering a range of energies, may further tend to reduce the efficiency of excitation, particularly where very narrow bandwidth laser radiation is employed, as in the case of selective photoexcitation, by having the laser radiation cover a more narrow range of frequencies than the width of the absorption line for the particles being photoexcited.